1. Field of the Invention
The present invention relates generally to a method and apparatus for the management and control of various flow streams related to the operation of a fuel cell engine. The present invention relates more specifically to a stream conditioning system that conditions at least one fuel cell reactant stream in communication with the fuel cell of a fuel cell engine. Namely, the present system can condition the temperature and humidity of a reactant stream supplied to, or exhausted from, either the fuel cell""s inlet and outlet streams, on either its anode or cathode side, or on both sides, as described in more detail below.
2. Description of Related Art
Fuel cells are electrochemical devices that convert a fuel""s energy directly to electrical energy. Fuel cells operate much like continuous batteries when supplied with fuel to the anode (negative electrode) and oxidant (e.g. air) to the cathode (positive electrode). Fuel cells forego the traditional extraction of energy in the form of combustion heat, conversion of heat energy to mechanical energy (as with a turbine), and finally turning mechanical energy into electricity (e.g. using a dynamo). Instead, fuel cells chemically combine the molecules of a fuel and oxidizer without burning, dispensing with the inefficiencies and pollution of traditional combustion.
The utility of fuel cells has been known since at least as early as 1939. Further developments in the fuel cell field have included the development of proton exchange membrane (PEM) fuel cells, phosphoric acid fuel cells, alkaline fuel cells, and fuel cells incorporating reformer technology to crack hydrocarbons such as gasoline to obtain hydrogen to feed the fuel cell.
Fuel cells generally require two independent flow circuits for delivering reactant streams to the anode and the cathode of the fuel cell. In PEM fuel cells, these flow circuits include an anode circuit for feeding the fuel, generally hydrogen, to the fuel cell; and a cathode circuit for feeding the oxidant, typically air from the ambient, to the fuel cell. In order to maintain proper operating conditions for the fuel cell, the temperatures and humidities of the anode and cathode circuits must be precisely controlled to avoid drying out the electrolyte or otherwise damaging the fuel cell, and thereby stopping the flow of electricity from the fuel cell.
Fuel cells have found application in a number of fields. Stationary cells are used in the utility industry and in commercial/residential settings. An area of particular interest has been the application of fuel cell technology in electrically-powered transport vehicles, including automobiles. In automotive applications, weight and space are at a premium, and therefore, the fuel cell and its supporting systems must be as small and lightweight as possible. Moreover, because automotive applications subject equipment to a wide and rapidly fluctuating range of operating conditions such as temperature and humidity, equipment utilized in such applications must be capable of withstanding and operating under a variety of conditions. Equipment utilized in automotive applications must also be sufficiently rugged to withstand the vibrations and stresses induced by over-the-road use.
Systems for conditioning the flow circuits of a fuel cell have been proposed. For example, U.S. Pat. No. 3,516,867 to Dankese discloses a fuel cell system including a dehumidifier and a humidifier for conditioning the fuel cell""s reactant streams. The humidification portion of this system achieves moisture transfer through a partition. This type of humidification system has been found inefficient for automotive applications, mainly because of the large surface areas required to transfer the necessary quantity of moisture to the reactant streams, and because of the undesirable weight of such large-scale systems. In addition, large quantities of heat energy are consumed in vaporizing moisture in this type of humidification system, which energy consumption reduces system efficiency.
U.S. Pat. No. 3,669,751 to Richman discloses a fuel cell, hydrogen generator, and heat exchanger system, wherein reactant air to be supplied to the fuel cell is brought into evaporative contact with a wet electrolyte to humidify the reactant air. The system of Richman suffers similar disadvantages to that of Dankese; namely, the requirement of large surface areas for effecting moisture transfer and the resulting weight of system components, as well as the consumption of considerable energy in vaporizing the moisture.
In addition to cathode humidification, existing fuel cell technology requires the humidification of the hydrogen fuel stream input to the fuel cell""s anode in order to prevent drying out the electrolyte within the fuel cell. This requirement of anode humidification adds additional components to a fuel cell""s gas management system, resulting in undesirable increased weight and expense. Moreover, known humidification systems such as membrane humidifiers or systems utilizing airflow through beds of wetted spheres consume considerable energy in vaporizing water to provide the required humidification. Therefore, it has been found that known methods of anode humidification are unsuited to automotive applications.
Thus it can be seen that a need yet exists for a lightweight, efficient means of conditioning fuel cell reactant streams of a fuel cell engine.
Yet another need exists for a stream conditioning system for conditioning the oxidant flow to the cathode inlet of a fuel cell.
A need further exists for a method and apparatus for conditioning the anode inlet to a fuel cell, which minimizes the weight and expense of associated components.
It is to the provision of such a fuel cell engine stream conditioning system meeting these and other needs that the present invention is primarily directed.
Briefly described, in a preferred form, the present invention is a stream conditioning system for a fuel cell gas management system or fuel cell engine. The stream conditioning system manages species potential in fuel cell reactant streams. A species transfer device is located in the path of at least one reactant stream of a fuel cell""s inlet or outlet, which transfer device conditions that stream to improve the efficiency of the fuel cell. A species transfer device can also be located in the paths of both the fuel cell""s inlet and outlet streams. In addition, the typical fuel cell has both an anode side and a cathode side, each side having reactant inlet and outlet streams. The conditioning system of the present invention can be used to condition a reactant stream on either the anode or cathode side, or both sides.
A reactant stream to be conditioned by the present species transfer device is defined as incorporating a potential of a species. Potential is used throughout as a term of relationship, describing the relative potential of a species between two or more streams. Potential will typically be used in context of one stream having a high potential of something (species), and another stream having a lower potential in that something (species) than the one stream, which context can be read as one stream having a higher potential of the species than does the other stream. Species is used throughout as a term describing a component of a stream that the transfer device seeks to remove from that stream, and transfer from that stream to another stream.
The present stream conditioning system communicates with at least two streams, one stream having a higher potential of a species than a lower potential stream. Yet, only one of the two streams need be in the flow path of the fuel cell; that is, only one stream need be a fuel cell reactant stream. The other of the streams can be an exhaust stream, or other such stream, provided to the conditioning system simply to enable transfer of the species from or to this other stream from or to the fuel cell reactant stream. A stream in the flow path of the fuel cell may be referred to as a reactant stream, as this stream will incorporate either a useful reactant for fuel cell operation (flowing into an inlet of the fuel cell), or will incorporate a by-product of a reactant being exhausted from the fuel cell (flowing out of an outlet of the fuel cell).
The present stream conditioning system is capable of removing a portion of a species from a high potential stream, and transferring the removed species into a low potential stream. In the context of a fuel cell engine, the stream conditioning system incorporating a species transfer device may be regarded in one sense as a removal system used to remove a portion of an unwanted species away from the engine. For example, the high potential stream can be a reactant stream in communication with the fuel cell, and the low potential stream can be an exhaust or purge stream not in communication with the fuel cell, wherein the unwanted or detrimental species is removed from the reactant stream and exhausted from the fuel cell engine via the low potential stream. Yet, even in this removal system embodiment, the species transfer device cannot simply remove the species from the reactant stream, but must then transfer the removed species to the exhaust/purge stream. Thus, when the ultimate function of the present system is that of removing a harmful species from the fuel cell engine, it still acts as a transfer system, not just a removal system.
If, on the other hand, both streams are in communication with the fuel cell, then the term xe2x80x9cspecies transfer devicexe2x80x9d is a more adequate moniker, since the present system removes a portion of a wanted species from one reactant stream, and transfers it into the other reactant stream. In this context, the species is preferably kept in the fuel cell engine, to be used again, and not removed away from the engine.
The species transfer device incorporates an exchange media and a sorbent. The exchange media can be, but need not be, in the form of a wheel. A sorbent is herein defined as a substance that has the ability to take up and hold species, as by absorption or adsorption. The sorbent is chosen for its particular characteristics vis-A-vis the species to be transferred. If the species transfer device is used to transfer latent heat between streams, the sorbent can be a desiccant. If the species is CO, then the sorbent is a substance that can collect and release CO between the streams.
As an example of the present stream conditioning system, the system can be a heat transfer system for a fuel cell engine. A first stream is defined as having a higher potential of heat than a second stream, wherein it is preferable to recapture some of the heat for reuse in the fuel cell engine, rather than having the heat simply exhausted away from the engine. Therefore, the present invention would be used both to remove a portion of the heat (latent, sensible or both) from the first stream, and to transfer the heat to the second stream, via the laws of thermodynamics. In this example, heat is the species, and the two streams differ in their potential of this species, so that heat is transferred by the present invention from a high potential stream to a low potential stream. The majority of sensible heat would be transferred via the exchange media of the heat transfer device, while the latent heat would be transferred via the sorbent, in this case, a desiccant.
As the present invention is a stream conditioning system for use in connection with a fuel cell engine, at least one stream conditioned by the present invention will be an inlet or outlet reactant stream of fuel cell. The species acted upon by the present stream conditioning system will be those types of species in reactant streams of a fuel cell. The most common species are latent heat, sensible heat, air, oxygen, hydrogen, nitrogen, carbon monoxide, carbon dioxide, sulfur dioxide and methanol. Specifically, the present invention can thus condition the cathode side inlet/outlet reactant streams, the anode side inlet/outlet reactant streams, cooling inlet/outlet streams, or a combination of these streams. Primarily, the present invention is used both to capture a portion of a species from an outlet (or exhaust) stream, and to transfer it back into an inlet stream in order to recover what would typically be wasted or lost in a fuel cell engine.
In another embodiment of the present stream conditioning system, the species transfer device can be in the flow path of three or more streams. For example, in an embodiment of a fuel cell engine having a fuel cell with a cathode loop and an anode loop, the cathode and anode exhaust streams can both reject a species into a species transfer device, to be transferred into a lower potential third stream.
As described, the preferable environment of the present stream conditioning system is in combination with a fuel cell engine. A fuel cell engine can comprise a cathode loop, an anode loop, and a cooling loop, each loop in communication with a fuel cell. The present invention can condition any one, or more than one, of the streams flowing through these loops. In such a fuel cell engine, a first reactant is typically circulated through the cathode loop. The first reactant is introduced to the fuel cell through an inlet, and is removed from the fuel cell through an exhaust. Similarly, a second reactant is circulated through the anode loop. The anode reactant is introduced to the fuel cell through an inlet, and is removed from the fuel cell through an exhaust. The cooling loop can circulate deionized water through the fuel cell via a cooling water inlet and a cooling water discharge, and is used to reject heat from the fuel cell. The cooling loop can alternatively circulate other liquids or gases that can remove heat from the fuel cell.
The cathode loop can include both a compressing means for pressurizing the cathode reactant to be input to the fuel cell""s cathode for use as the fuel cell""s oxidant, and the present conditioning system for conditioning the pressurized cathode reactant. The compressing means, however, may not be necessary in all circumstances.
The present species transfer device used in connection with the cathode loop can be in the embodiment of a sensible and latent heat transfer device. The sensible and latent heat transfer device transfers sensible and/or latent heat from the cathode reactant""s inlet or outlet stream to the reactant""s other of the inlet or outlet stream. The direction of transfer of these species, sensible and latent heat will, as described above, depend on which stream has a higher potential of sensible and/or latent heat. xe2x80x9cAnd/orxe2x80x9d is used here because it is possible that the present system may transfer one such species from a reactant""s inlet stream to a reactant""s outlet stream, while concurrently transferring another such species from the outlet stream to the inlet stream. For example, a fuel cell cathode reactant inlet stream may have a lower potential of latent heat and a higher potential of sensible heat than those of the cathode reactant outlet stream. As such, the sensible and latent heat transfer device embodiment of the present invention would transfer a portion of the cathode reactant outlet stream""s latent heat to the cathode reactant inlet stream, and would transfer a portion of the cathode reactant inlet stream""s sensible heat to the cathode reactant outlet stream.
If the cathode loop of the fuel cell engine further incorporates a turbo charger, the sensible and latent heat transfer device of the present system can operate to transfer each of these two species in opposite directions, as described above. The sensible heat transferred from the inlet to the exhaust, and the latent heat transferred from the exhaust to the inlet.
This sensible and latent heat transfer device embodiment of the present stream conditioning system can include an exchange media in the form of an enthalpy wheel, and the sorbent in the form of a desiccant. Depending on stream potentials, the enthalpy wheel can operate by removing both sensible and latent heat from the fuel cell cathode reactant exhaust stream to heat and humidify the cathode reactant inlet stream. Because the water vapor adsorbed from the cathode reactant exhaust is desorbed into the cathode reactant inlet stream, it is unnecessary to provide external energy (in the form of heat of vaporization) in transferring moisture removed from the cathode reactant exhaust to the cathode reactant inlet by the enthalpy wheel.
The desiccant of the sensible and latent heat transfer device is capable of removing moisture through adsorption from the cathode reactant exhaust stream, which removal releases heat and raises the exhaust stream temperature, which in turn heats the media of the enthalpy wheel. This same heat is used to power the desorbtion phase in the cathode reactant inlet stream upon rotation of the enthalpy wheel. A desiccant material naturally attracts moisture from gases and liquids. The material becomes saturated as moisture is adsorbed or collects on the surface; but when exposed to a dryer stream, the desiccant gives up its moisturexe2x80x94or regeneratesxe2x80x94and can be used again. The enthalpy wheel of the present invention can include solid desiccants, for example, silica gel, activated alumina, lithium chloride salt, and molecular sieves. Titanium silicate, a class of material called Im, and synthetic polymers are newer solid desiccant materials designed to be more effective for cooling applications. Alternatively, the enthalpy wheel can include liquid desiccants, for example, lithium chloride, lithium bromide, calcium chloride, and triethylene glycol solutions.
Although the enthalpy wheel embodiment is preferred, the species transfer device can alternatively comprise two or more beds or towers of sorbent material, operated by means of continuous sequential valving, to alternate between a charging mode whereby a fuel cell exhaust stream heats and humidifies a desiccant, and a discharging mode whereby the heat and humidity collected by the desiccant beds or towers are released into the cathode inlet stream.
As the present stream conditioning system is provided to transfer a species from a stream with a high potential of that species to a stream with a lower potential of that species, at least one stream being in communication with a fuel cell, the size of the pores in the exchange media of the wheel can be varied, and the sorbent of the transfer device can be chosen to selectively filter out different types of species. For example, in an enthalpy wheel used in connection with the cathode loop of the fuel cell engine, the sorbent can be a desiccant to transfer moisture from the exhaust stream to the inlet stream. Alternatively, the sorbent can be sized to filter nitrogen and other species of the cathode reactant inlet stream, thereby increasing the partial pressure of oxygen in the inlet stream to increase fuel cell efficiency. Further, two or more enthalpy wheels can be provided in series, each of which having pores sizes to selectively filter various species from the cathode inlet and/or exhaust streams. In yet another embodiment, a single enthalpy wheel can incorporate more than one sorbent, each sorbent capable of filtering a different species.
The cathode loop of the fuel cell engine can further comprise an adiabatic quench to cool the cathode reactant inlet stream. The adiabatic quench controls the dry bulb temperature of the inlet stream by vaporization of quench water into the inlet stream, either prior to its introduction to a sensible and latent heat transfer device, or, alternatively, upon its exit from a heat transfer device. The adiabatic quench can comprise collecting means for collecting liquid from the cathode exhaust stream, transfer means for transporting the collected liquid to the point of introduction to the cathode inlet stream, and an introducing means to introduce the collected liquid into the cathode reactant inlet stream. The introducing means can comprise an ultrasonic nozzle for dispersing the liquid, in a mist of fine droplets, into the inlet stream.
In such an engine, the quench rate at the cathode inlet controls the dry bulb temperature of the cathode reactant inlet stream, while the speed of the enthalpy wheel controls the dew point temperature, and thus the relative humidity, of the inlet stream. By varying the speed of the rotation of the enthalpy wheel, the amount of moisture transferred to the cathode inlet can be varied. Temperature, pressure, and relative humidity sensors can be provided to monitor the cathode inlet stream conditions and provide feedback control, through a computerized control system, for the adiabatic quench rate and the rotational speed of the enthalpy wheel.
In some cases, what would normally be the cathode reactant inlet and outlet streams are instead sent to the anode side of a fuel cell. The species transfer device for the cathode reactant inlet and outlet described above is equally applicable to the case where such reactant is sent instead to the anode side.
An anode loop of a PEM fuel cell engine can comprise one or more eductors or other means for recirculating the anode reactant exhaust stream to the anode inlet. By mixing the anode exhaust with supply hydrogen from storage tanks, the cool, dry hydrogen from the tanks is humidified by an approximately equal amount of moist exhaust hydrogen from the fuel cell. The fuel cell operating conditions are controlled to provide an excess of hydrogen (preferably at a stoichiometric ratio of approximately 2.0) to the anode inlet stream, and to control the temperature of the anode inlet hydrogen stream. In this manner, the total enthalpy of the anode exhaust stream is controlled in relation to the inlet total enthalpy, thereby, in effect, utilizing surplus hydrogen to transport anode humidity through the fuel cell, back to the anode inlet stream and preventing moisture from condensing out of the anode reactant stream in the fuel cell. In this manner, the need for an anode humidifier and its associated equipment is eliminated, thereby reducing weight, expense and occupied space. The anode loop also functions to maintain the anode pressure at or near that of the cathode in order to minimize the possibility of blowing out the cell""s membranes.
The anode loop can further comprise another embodiment of the present stream conditioning system, a contaminant sweep. The contaminant sweep is another type of species transfer device, for example, a CO sweep, which helps remove CO from the anode reactant to a PEM fuel cell engine, as CO is bad for certain catalysts in the fuel cell, including platinum. Thus, in this embodiment, the present stream conditioning system can comprise an exchange media with a sorbent that is sized to remove CO from the anode reactant. Here, the system is not so much interested in transferring the CO between anode inlet and outlet streams, but in removing CO from the anode loop. Yet, the exchange media with a sorbent can only remove a species if it can move it from a high potential stream to a low potential stream. So, in effect, the present system can remove CO from the anode reactant stream if it can transfer it to another stream having a lower potential or concentration of CO.
Other contaminant sweep embodiments of the present invention can be located along any loop of the fuel cell engine, and can include, without limitation, a methanol sweep in the cathode loop to capture fugitive methanol from a direct methanol fuel cell engine, a nitrogen sweep to capture nitrogen from the cathode inlet stream, and a reformer product gas sweep to capture CO2 from the reformer outlet in a fuel cell engine incorporating a reformer. It will be understood that while the fuel cell engine can incorporate a methanol sweep to transfer methanol away from the engine and into, for example, the atmosphere, the preferable methanol sweep would be better termed a methanol catalyzer, described hereinafter.
The fuel cell, sometimes also referred to as a xe2x80x9cstack,xe2x80x9d is cooled by circulating deionized water through the stack by the cooling water loop. Deionized water is an aggressive corrosive agent and, therefore, stainless steel piping and equipment must be utilized in handling this deionized water. Because stainless steel is a poor thermal conductor, and is heavy and expensive, use of a stainless radiator to effect water-to-air heat transfer has been found to be undesirable, especially in automotive applications. Therefore, the fuel cell engine can utilize a stainless brazed plate heat exchanger to effect liquid-to-liquid transfer of stack heat, from a closed deionized water circuit to an ordinary ethylene glycol and water cooling stream. Then, a standard commercial automotive radiator system can be utilized for water-to-air heat transfer from the glycol/water stream. In this manner, heat from the deionized water is transferred by water-to-water convection through the brazed plate heat exchanger""s thin stainless plates, at a much higher heat transfer rate than could be obtained through stainless steel-to-air heat exchange by a stainless radiator. This aspect of the fuel cell engine enables the use of a more efficient, lighter, cheaper, aluminum water-to-air automotive radiator, and minimizes the quantity of deionized water required. The use of a more efficient aluminum radiator also reduces the required surface area for effecting heat transfer, thereby minimizing aerodynamic drag associated with the radiator.
The present stream conditioning system preferably is used with fuel cell engines related to both transportation (mobile) and stationary applications. The fuel cells used in connection with such applications are generally PEM fuel cells. Yet, it will be understood that the present stream conditioning system can be used in connection with other fuel cell types, including phosphoric acid, carbonate and solid oxide that, instead of the ion exchange membrane of the PEM, use phosphoric acid, alkali carbonate mixtures and yttria stabilized zirconia, respectfully, as the electrolyte.
The conditioning system of the present invention can be used in connection with reactant streams of PEM fuel cells that are direct methanol fuel cells (DMFCs). A DMFC can use a liquid methanol fuel feed, which eliminates the complexity and weight penalties of a reformer generally used in a fuel cell engine. DMFCs are a relatively new member of the fuel cell family. These cells are similar to the PEM cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120-190 degrees F. Higher efficiencies are achieved at higher temperatures.
For example, a stream conditioning system of the present invention used in connection with a DMFC engine can be a methanol catalyzer to catalyze fugitive methanol on the cathode side of the DMFC that migrates through the fuel cell from the anode side. The present system can convert the methanol to heat via a catalyst, and transfer the heat back into the cathode inlet stream. A catalyst can be included in the enthalpy wheel in the cathode loop to catch the fugitive methanol. The catalyst can be mixed with the sorbent. The fugitive methanol is catalyzed by the enthalpy wheel, generating heat that is reused. Unlike cracking the methanol with a catalytic converter that wastes the heat of cracking, use of the methanol sweep embodiment of the present system recirculates that heat, producing a more efficient DMFC engine. Using an enthalpy wheel embodiment of the present invention is particularly advantageous in a DMFC as the cell temperature must be fairly high to crack the methanol.
The fuel cell engine in which the present stream conditioning system operates can be a low pressure fuel cell engine embodiment, wherein the cathode loop can further comprise a pre-heater to add heat to the cathode reactant stream, which pre-heater can replace the adiabatic quench, as in this low pressure fuel cell engine embodiment, there is little to no heat of compression to remove from the cathode reactant stream. Conversely, in a high pressure fuel cell engine embodiment, if the compressing means provides a compression ratio of, for example, greater than or equal to 3:1, then the cathode loop can incorporate the adiabatic quench that alone acts as a conversion device, converting sensible heat to latent heat, without need to resort to an enthalpy wheel of the present invention, as the quench water is enough to humidify the cathode reactant stream.
Accordingly, it is an object of the present invention to provide a stream conditioning system for conditioning at least one reactant stream of a fuel cell, which system is compact, lightweight and inexpensive.
Another object of the present invention is to provide a stream conditioning system that enables cathode air humidification and anode hydrogen humidity retention for a fuel cell engine.
A further object of the present invention is to provide a method and apparatus for transferring sensible and latent heat from the fuel cell""s cathode exhaust stream to the cathode inlet stream.
Yet another object of the present invention is to provide a method and apparatus for retaining humidity within the hydrogen fuel stream supplied to the anode inlet of a fuel cell.
These and other objects, features, and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.